1. Field of the Invention
The invention concerns a method to generate spatially resolved, quasi-T2-weighted magnetic resonance signals, as well as a device to implement such a method. Either quasi-T2-weighted image data are then estimated from the quasi-T2-weighted magnetic resonance signals, or the T2 relaxation constant is estimated with spatial resolution.
2. Description of the Prior Art
Magnetic resonance imaging has found broad application in the medical field. One of the advantages is an excellent contrast in the imaging of different tissue types and even bodily fluids. The contrast response is determined by the varying longitudinal and transverse relaxation constants T1 and T2 and T2* of the different tissue types and body fluids, as well as by the sequence type used for signal acquisition and the repetition time TR and echo time TE that can be set.
In recent times, edema imaging of the heart has gained increasing clinical importance. For example, edema imaging allows a fresh infarct to be characterized. In magnetic resonance imaging, an edema in the myocardium is characterized by extended longitudinal and transverse relaxation constants T1 and T2 relative to healthy myocardium. Edema imaging is concerned primarily with utilizing this extension in order to optimize the contrast. Since the myocardium moves and there are only short periods of relative rest in the cardiac cycle, the measurement method must be fast and robust with regard to movement.
Two different sequence types have previously been used for edema imaging. In both sequence types, a triggered signal acquisition in a segmented raw data matrix takes place over multiple heart beats in the respective phases of relative rest in each cardiac cycle. In this case, k-space or the raw data matrix is segmented corresponding to the number of cardiac cycles used for signal acquisition.
In principle, the use of a turbo spin echo sequence (TSE sequence) with a T2-weighted reordering is suggested as a first sequence type. However, the TSE sequence is relatively susceptible to movement.
The second sequence type that is used is a Trufi sequence. The Trufi method represents a balanced Steady State Free Precession (bSSFP) sequence. In a first variant, the Trufi sequence is incorporated into a hybrid method and uses a T2 preparation module. The subsequent signal acquisition takes place according to the Trufi method. In a second variant, the Trufi signal acquisition takes place after excitation with a 180° pulse series. This hybrid method is described in the article by Aletras et al.: “ACUT2E TSE-SSFP: A Hybrid Method for T2-Weighted Imaging of Edema in the Heart”, Magnetic Resonance in Medicine (2008), Vol. 59, Pages 229-235. A T2-weighted representation of the myocardium can be generated with this method. The signal-to-noise ratio and the contrast-to-noise ratio in the myocardium are similar to those in a TSE sequence. This hybrid method places high demands on the capacity of the radio-frequency transmitter and causes a high SAR exposure for the patient. The application of this hybrid sequence is limited because the T2 preparation pulse is susceptible to inhomogeneities of the basic magnetic field. The Trufi signal acquisition additionally generates a transient contrast: the optimal edema contrast at the beginning of the sequence transitions over the course of the signal acquisition (given a steady state) to a contrast that is determined by T1 and T2, more precisely by the quotient T1/T2. In order to keep the contributions of the unwanted T1 contrast as small as possible, it is sought to use a flip angle of 180° after the first excitation.
The T2 contrast response of CPMG-like sequences is described in the article by M. D. Hürlimann and D. D. Griffin entitled “Spin Dynamics of Carr-Purcell-Meiboom-Gill-like Sequences in Grossly Inhomogeneous B0 and B1 Fields and Application to NMR Well Logging”, in the Journal of Magnetic Resonance (2000), Volume 143, pages 120 to 135.